Specify compiler host server through environment variable for bisection tool

[Andi]

Summary

The bisection tool goes to a specific URL by default to fetch compilers.
This patch allows a user to specify an environment variable which then contains the URL from which the bisection tool tries to fetch the compilers.

[cmatthews]

Looks good. Thanks Andi!

zorg/trunk/llvmbisect/llvmlab/gcs.py

 """Integration with Google Cloud Storage.
 
 """
 import os
 import requests
 import urllib3
 
 # Root URL to use for our queries.
-GCS = "https://www.googleapis.com/storage/v1/"
+DEFAULT_GCS = "https://www.googleapis.com/storage/v1/"
+
+# Specify a different root URL if compilers are hosted elsewhere.
+GCS = os.getenv("GCS_SERVER", DEFAULT_GCS)
 
 DEFAULT_BUCKET = "llvm-build-artifacts"
 
 BUCKET = os.getenv("BUCKET", DEFAULT_BUCKET)
 
 class HttpClient(object):
     def __init__(self):
         self.session = requests.Session()